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This image shows Pluto (lower left) and its large moon Charon (upper right). Notice how large Charon is compared to Pluto, and how close the moon is to the planet. This image was taken by the Hubble Space Telescope on February 21, 1994.
Click on image for full size Image courtesy Dr. R. Albrecht (ESA/ESO Space Telescope European Coordinating Facility) and NASA.

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Charon - largest moon of Pluto

Charon is by far the largest of Pluto's
known moons. Charon was discovered
by the American astronomer James Christy on June 22, 1978, while working at
the U.S. Naval Observatory.

Charon is so large compared to Pluto that
the pair are often referred to as a "binary
planet" (double planet). The radius of Charon is 603.6
km (375 miles), which is about half that of
Pluto. The moon also orbits very
close to the planet, just over 1/20th the distance between Earth and
the Moon.
Charon circles Pluto once every 6.4 days at a distance of 19,571 km (12,161
miles).
Like Earth's
moon,
Charon
rotates
exactly
once
per orbit, so the same side of the moon always faces the planet. Unlike the
Earth-Moon
system,
this "orbital lock" phenomenon also extends to Pluto; Pluto's
day is exactly as long as the orbital period of Charon. This means that
Charon is always in the sky when viewed from one hemisphere of Pluto, while
an observer
standing on the other side of Pluto would never see Charon!

Some astronomers think Charon may have formed in a way similar to the
process that created Earth's moon. They think a
large object slammed into Pluto early
in its history, and that the debris from this collision coalesced to form the
moon. Other astronomers disagree, citing the nearly identical densities of
Pluto and Charon as evidence which conflicts with this formation theory. According
to the theory, Charon should be less dense than Pluto, since it should be made
up of material from the outer layers of
the planet and not from the denser
material at the planet's core. Charon is composed
of rock and ice; a little
less than 2/3rds rock and slightly more than 1/3rd ice.

Charon is named after a character from Greek
mythology. Charon was the ferryman
of the dead, who conveyed the recently deceased across a river into the underworld.
Hades, the Greek god of the underworld, was called Pluto
in Roman mythology.
Although the "ch" in the name Charon is pronounced like a "k" in
Greek, most astronomers pronounce the moon's name with a "sh" sound
at the start. That is because James Christy, the discoverer of Charon, chose
to pronounce it that way in honor of his wife Charlene.

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